1. Field of Invention
The present invention relates to the processing of signals produced by inductive vehicle detectors, and more particularly to the normalization of such signals such that the same vehicle is recognized by different detectors or by the same detector at a different times.
2. Description of the Related Art
In inductive vehicle detectors of the prior art it is common practice to use manual switches to select the “frequency” and “sensitivity” of an inductive vehicle detector. Typical sensitivity settings are implemented as a threshold value that is offset from a baseline value by a fixed amount usually expressed in units of percent change in inductance. A vehicle is considered to have been detected when the inductance measurement output of the detector deviates from the baseline value by an amount greater than or equal to the threshold value. Inductive vehicle detectors generally have signature outputs which are typically digitized representations of an analog waveform corresponding to measured inductance versus time, and they generally have bivalent outputs which indicate the instantaneous presence or absence of a vehicle.
Typically, the baseline value is automatically adjusted instantaneously on power-up or reset, and adjusted incrementally in response to environmental drift; while the sensitivity threshold value is only adjusted manually. This leads directly to repeatability errors in presence, speed, length, occupancy and acceleration measurements which are based on the bivalent output of the detector.
It is not known in the prior art to calibrate either the signature output or threshold values of an inductive vehicle detector so that variations in the electrical parameters of the wire-loop and lead-line circuits from one detector to another, or time-varying environmental parameters for any particular loop-lead circuit, will cause a reduced repeatability error, from one detector to another, in the detector output.